In the design and manufacture of semiconductor IC (integrated circuit) chip packages and modules (e.g., SCM (single chip modules) or MCMs (multi-chip modules)), it is imperative to implement mechanisms that can effectively remove heat generated by IC chip devices (such as microprocessors) to ensure continued reliable operation of the devices. Effective heat removal becomes increasingly problematic as chip geometries are scaled down and operating speeds are increased, which results in increased power density. Although improved IC chip module designs are being developed to operate at higher clock frequencies, increased system performance is limited primarily by the ability to implement effective heat removal mechanisms to cool the IC chip modules.
Various heat removal techniques have been developed for cooling semiconductor packages. For example, one technique that is typically employed includes thermally coupling a thermal hat or package cap/lid/cover to one or more semiconductor IC chips using a compliant thermally conductive material (e.g., thermally conductive paste) as a thermal interface, and then thermally coupling the thermal hat or package lid to a cooling plate or heat sink. The package lid or thermal hat, which can be formed of a high thermal conductivity material, such as copper or aluminum, will conduct heat away from the IC chip(s) and the heat is removed from the cooling plate or heat sink by methods such as forced air cooling or circulating liquid coolants.
A compliant thermally conductive material (or TIM (thermal interface material)) is typically used (as opposed to a rigid bond) to thermally couple an IC chip to a thermal hat when, for example, the difference in thermal expansion between the material of the IC chip and the material of the thermal hat is relatively large. For example, there is a significant difference between the thermal expansion of a thermal hat made of copper (Cu) which has a thermal coefficient of expansion (TCE) of about 16.5 ppm/° C., and a silicon (Si) chip which has a TCE of about 2.5 ppm/° C. The use of a compliant thermally conductive material layer between a Cu thermal hat and a Si IC chip reduces stress at the thermal interface due to differences in thermal expansion of the IC chip and the thermal hat.
Moreover, effective cooling techniques are even more problematic for, e.g., a multi-chip module (MCM), wherein an array of chips mounted on a common substrate are thermally coupled to a common thermal hat or package lid, for example, using a compliant thermally conductive material. Indeed, differences in thermal expansion between the materials that form the package substrate, the chips, and the thermal hat, for example, can result in both vertical and horizontal deflections during power or temperature cycling. These deflections can lead to the migration of the compliant thermally conductive material out of the gap between the IC chips and the thermal hat, resulting in voids that increase the thermal resistance between the IC chip and the thermal hat and causing local increases in the operating temperature of the IC chips.
More specifically, when the TIM layers are compressed (due to thermal expansion of the chips and the thermal hat), by conservation of volume, some compliant material is squeezed out from between the chips and the thermal hat at the edges of the chips. When the pressure is relaxed, if the thermally conductive compliant material has not flowed too far past the edges of the chips, the compliant material will flow back into the gap between the chip and the thermal hat. If the compression force has caused the thermally conductive compliant material to flow beyond a certain critical distance beyond the gap between the chip and the thermal hat, air will flow back into the gap instead of the thermally conductive compliant material, thereby forming a void. As noted above, such voids can cause a significant increase in the chip operating temperatures in the regions of the chip in contact with the voids.
Furthermore, difficulties arise for efficient heat removal with respect to IC chips, such as processors, that have “hot spot” regions, which can have a heat flux significantly greater than the average heat flux, resulting in temperatures significantly hotter than the average chip temperature. A thermal solution that may be adequate for efficiently removing heat from a region of a chip having an average chip power density may not be adequate for removing heat from a “hot spot” region of the chip, i.e., a region having an above average chip power density, which can result in failure of the chip devices within or near the “hot spot” region.